It is well-known in the art to retrieve, and sometimes to store, data using laser light or electromagnetic waves near the light spectrum as part of the process. This is common in CD players, in DVD players and DVD recorders, and in computers. For example, a typical optical disc drive for reading data stored on an optical disc includes a light emitter such as a semiconductor laser, photodiodes for detecting the light reflected from the disc's surface, and means for directing light beams to and receiving them back from particular points on the surface of the disc. As examples, CD lasers are common with an infrared wavelength of about 780 nm. DVDs commonly use a red wavelength of about 650 nm. Blu-Ray Discs commonly use a violet wavelength of about 405 nm. All electromagnetic waves used to store data on or retrieve data from an optical storage medium are referred to herein as light beams.
As an example of how the data is stored, depressions are made at particular points on the surface of a disc to depths amounting to particular fractions of a wavelength. When retrieving the data, the reflected light beam's phase is shifted with respect to the phase of the incoming light beam, and the mutual interference affects the intensity of the reflected light beam and, correspondingly, the electrical signals output from the photodiodes. As another example of how data is stored, particular points of a dye layer on a disc are selectively heated with a laser light beam, changing the reflectivity of the dye and, correspondingly, the intensity of the reflected light beams and resulting electrical signals from the photodiodes when data is being retrieved. As another example of how data is stored, the intensity and timing of a laser light beam applied to particular points of a crystalline metal alloy layer on a disc affect their form (e.g. crystalline or amorphous) and reflectivity.
The means mentioned above for directing light beams to and receiving them back from particular points on the surface of an optical storage medium may include software and electrical, mechanical and optical components.